memoize

[Andrei Alexandrescu]

On 1/4/11 7:59 AM, Simen kjaeraas wrote:
> Guilherme Vieira <n2.nitrogen at gmail.com> wrote:
>
>> @Walter: would it be hard/impossible for the compiler to look at
>> memoize and
>> tell it exhibits pure behavior and is, thus, pure?
>
> The simplest solution is this:
>
> template memoize( alias fn ) {
> static if ( isPure!fn ) {
> pure auto memoize( ParameterTypeTuple!fn ) {
> // Blah!
> }
> } else {
> auto memoize( ParameterTypeTuple!fn ) {
> // Blah!
> }
>
> }
> }

I've been mulling over this for a while, maybe it's time to start 
discussing it.

Often you want to say "this entity is pure/const/immutable/safe... if 
this other entity is the same, or generally if this Boolean is true". So 
I was thinking of introducing e.g. a constrained pure:

template memoize( alias fn ) {
     pure(isPure!fn) auto memoize( ParameterTypeTuple!fn ) {
         ...
     }
}

So generally when you write "attribute(expression)" the attribute will 
be in effect if and only if the expression is true.

D has become very powerful at introspecting most of its own abstractions 
in the form of compile-time Booleans. This language extension would 
close the circle by allowing D to introduce attributes and qualifiers 
depending on compile-time Booleans.


Andrei